Introduction - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 5

conveyor belt systems all break up the coal, producing quantities of fine coal
particles whose recovery requires special processing and cleaning techniques.
The characteristics of the in-situ coal (e.g., ash and sulfur content) vary
spatially. The product that emerges from a mine often includes, not only
randomly distributed impurities and the non-coal material known as partings
(see Sidebar 1.2 for a description of the processes that deposit coal and create
the impurities), but also material from the roof and floor layers. Thus, the
characteristics of mined coal vary considerably.
End-use plants are engineered for optimal combustion to burn feedstock of
a particular ash, sulfur, and energy content—requirements rarely met by the run-
of-mine coal or by coal from a single source ( Figure 1.2 ). These combustion
requirements have been the impetus for all upstream process changes, including
the search for coal of low sulfur content, improved coarse and fine coal cleaning
processes, in addition to several disposal and environmental laws.
SIDEBAR 1.2 COAL GEOLOGY
Coal is a combustible material consisting of organic matter and minor
amounts of inorganic materials. It is derived from a heterogeneous
mixture of plant remains and associated minerals, which have undergone
chemical and physical changes by geological and biological processes
without free access to air. Coal has a highly variable composition,
affecting both its chemical and physical properties. Except for the
anthracite region in eastern Pennsylvania, coal beds usually occur as
nearly horizontal or gently folded strata. A coal seam is a composite of
several layers, each of which may consist of a different mixture of coal
material and mineral matter. Occasionally, these layers may be
completely inorganic, such as shale, or high in mineral matter. Such
layers are referred to respectively as partings and bony coal. Coal and
associated rocks may contain significant amounts of sulfur, arsenic, and
other materials whose presence in the waste engenders environmental
concerns.
The depositional environments that produced the coalfields of the
Eastern United States were predominantly coastal-deltaic. Large volumes
of sediment were deposited from the Appalachian Mountains into rivers,
which emptied into bays and coastal seaways. These conditions
engendered the development of vast, laterally extensive peat swamps
along the coastlines and delta platforms. Periodically, sea level rose and
shallow marine environments flooded the swamps, depositing marine
shales and limestones. The cyclic repetition of these sedimentary
environments has resulted in some of the most complex stratigraphic
sequences in the geologic record.
SOURCE: Rice et al., 1979.